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REVIEW ARTICLE Soil- and waterborne Phytophthora species linked to recent outbreaks in Northern California restoration sites A review identifes several Phytophthora species found in California wildlands and discusses approaches for preventing and diagnosing the spread of these plant pathogens. by Matteo Garbelotto, Susan J. Frankel and Bruno Scanu istorically, the release of Phytophthora species Abstract in the wild has resulted in massive die-ofs of Himportant native plant species, with cascading Many studies around the globe have identifed plant production facilities consequences on the health and productivity of afected as major sources of plant pathogens that may be released in the wild, ecosystems (Brasier et al. 2004; Hansen 2000; Jung with signifcant consequences for the health and integrity of natural 2009; Lowe 2000; Rizzo and Garbelotto 2003; Swiecki ecosystems. Recently, a large number of soilborne and waterborne et al. 2003; Weste and Marks 1987). Once introduced, species belonging to the plant pathogenic genus Phytophthora have plant pathogens in general cannot be eradicated (Cun- been identifed for the frst time in California native plant production nife et al. 2016; Garbelotto 2008), and costs associated facilities, including those focused on the production of plant stock used with the spread and control of exotic pathogens and in ecological restoration eforts. Additionally, the same Phytophthora pests have been estimated to surpass $100 billion per species present in production facilities have often been identifed in failing year for the United States alone (Pimentel et al. 2005). restoration projects, further endangering plant species already threatened Tus, preventing the introduction of pathogens by us- or endangered. To our knowledge, the identifcation of Phytophthora ing pathogen-free plant stock is the most cost-efective species in restoration areas and in plant production facilities that produce and responsible approach (Parnell et al. 2017). plant stock for restoration projects is a novel discovery that fnds many In their extensive meta-analysis, Santini et al. land managers unprepared, due to a lack of previous experience with (2013) identify the trade of live plants as the main these pathogens. This review summarizes some of the key knowledge pathway for the introduction of invasive forest dis- about the genus Phytophthora in general and lists some of the many eases in Europe. Similarly, Jung et al. (2016) identi- soilborne and waterborne species recently recovered from some California fed plant production facilities as a major source of restoration sites and plant production facilities. Online: https://doi.org/10.3733/ca.2018a0033 Phytophthora diseases are increasingly being found in California wildlands and parks, where they have caused large die -ofs of native plant species. Shown here is Ione manzanita (Arctostaphylos myrtifolia) in Ione, California, killed by P. cinnamomi. Matteo Garbelotto Matteo 208 CALIFORNIA AGRICULTURE • VOLUME 72, NUMBER 4 Phytophthora inoculum that may be released in the genus Phytophthora is part of the order Peronosporales: wild. Te best-known example of a Phytophthora spe- this order contains genera that are notable for having cies released in California natural environments from co-evolved with plant hosts mostly as plant pathogens, commercially produced plants is that of Phytophthora although some are pathogens of animals (Spies et al. ramorum (Grünwald et al. 2012), but an equally im- 2016; Tines 2014). Te four best-known genera are portant prior introduction associated with infested Peronospora, Plasmopara, Pythium and Phytophthora. plant nurseries is that of Phytophthora lateralis, which Each has evolved distinct epidemiological strategies. afected Port Orford cedar in California and Oregon While Peronospora and Plasmopara species (causal (Hansen et al. 2000). agents of plant diseases known as “downy mildews”) Recently, Rooney-Latham and colleagues (Rooney- mostly spread aerially, Pythium species are almost ex- Latham and Blomquist 2014; Rooney-Latham et al. clusively soilborne and waterborne. Te genus Phytoph- 2015) identifed at least two soilborne Phytophthora thora stands between the two, and includes species that species, including one reported for the frst time ever are soilborne and waterborne, or airborne, and some in the United States, as the cause of extensive mortality species with a mixed epidemiological strategy (Bourret of two plant species recently employed in an extensive et al. 2018; Oßwald et al. 2014). restoration project. Both species were also found in the Phytophthora propagules responsible for much of production facilities that had supplied the plant stock, the known host-to-host spread are normally ovoid or and both species have been shown, through greenhouse pyriform in shape and are called sporangia (fg. 1A). inoculation studies, to be aggressive pathogens on three Sporangia can be extremely variable in form and size important hosts present in the restoration areas (Sims and are normally produced alone or in clusters at the et al. 2018). Tis discovery triggered multiple surveys of end of stalks. If sporangia can be easily detached from failed restoration projects and of the facilities that pro- the stalks that bear them, the species may be aerially vided plants employed in such projects (Frankel et al. dispersed rather than just being soilborne and/or wa- 2018). While soilborne and waterborne Phytophthoras terborne (Erwin and Ribeiro 1996). have been found in commercial production of orchard Sporangia of all Phytophthora species, when mature, and landscaping plants, to our knowledge this is the contain a variable number of motile, bifagellate zoo- frst reported case of Phytophthora species found in spores (fg. 1B). Sporangia sometimes can germinate plants bound for native landscapes (Frankel et al. directly and infect a plant, or plants can be infected 2018; M. Garbelotto, unpublished results). Although directly by hyphae growing in the soil. However, it is Phytophthora species are known to be plentiful in com- the zoospores that are mostly responsible for infec- mercial plant production facilities, their discovery in tion of plant tissue. Zoospores are normally attracted native plant production facilities is novel, and fnds by chemical or electrical signals generated by the many land managers unprepared, due to a lack of pre- plant host (Carlile 1983) and require a flm of water to vious experience with these pathogens. “swim” and initiate the infection process. If there is no Given that the research community has been flm of water or water dries out, zoospores can encyst focused on aerial Phytophthora species such as P. and become dormant without losing viability. Infection ramorum recently, this review summarizes some basic by zoospores or by germinating sporangia can occur knowledge for soilborne and waterborne Phytophthora through stomatal openings, or an infection peg can species, such as those recently recovered from restora- rupture the plant cell wall and directly infect plant tis- tion and disturbed sites in the San Francisco Bay Area sue (Erwin and Ribeiro 1996). Te need for a flm of in California. Even if we acknowledge that infected water for zoospore-mediated infection to occur largely plants can ofen be asymptomatic (Bienapf and Balci explains the direct relationship between increasing dis- 2014; Jung et al. 2016; Migliorini et al. 2015), we hope ease levels and increasing rainfall values. this article may increase the awareness about this Phytophthora species also produce spherical sur- group of pathogens, possibly leading to their early vival structures called chlamydospores (fg. 1C). Te detection in plant production facilities (Parke et al. size of chlamydospores, the pattern and the abundance 2014; Patel et al. 2016), before infected plants are out- in which they are produced, and the thickness of their planted in the wild. outer wall can ofen be diagnostic traits diferentiating Phytophthora species. Chlamydospores can survive Introduction to the genus up to several years in adverse environmental condi- Phytophthora tions; they can also contaminate soil and water and be responsible for dispersal of the pathogen. In favorable For decades, Phytophthora species have been errone- conditions, chlamydospores can germinate directly ously lumped with the Fungi, but in order to fully or they can produce a sporangium. Like sporangia, understand their biology and ecology it is important chlamydospores are clonally produced and do not to understand their correct taxonomic position. Te require mating. genus Phytophthora belongs to the kingdom Stramini- Sexual structures produced by Phytophthora spe- pila (formerly Chromista), which also includes aquatic cies afer mating are called oospores and are produced organisms such as diatoms and kelp (Dick 2001). Te by a single individual in homothallic species, or when http://calag.ucanr.edu • OCTOBER – DECEMBER 2018 209 two individuals bearing diferent mating types come chlamydospores (fg. 1D). Note that oospores of ho- into contact in heterothallic species. Exposure of het- mothallic species will be genetically identical to the erothallic species to certain fungi or chemicals can individual that produced them, because recombination also trigger the formation of oospores in the absence between homologous chromosomes cannot generate of mating (Pratt et al. 1972; Uchida and Aragaki variation, while oospores of heterothallic species will 1980). Oospores are particularly thick walled and